The availability of integrated-circuit, alarm clocks having very low voltage and current requirements has made possible the design of a solid state, timer-controlled thermostat which in the presence of smoke, sounds an alarm and inhibits the application of power to a heat exchanger under its control. It replaces conventional, bimetallic-strip thermostats with out the installation of additional wiring to satisfy power requirements and, therefore, can be installed by a person who is not a skilled electrician.
The application of power to the heat exchanger is determined by the on or off state of a silicon controlled rectifier which is controlled by supplying voltage to its gate from a voltage divider comprised of a variable resistor and a thermistor. The temperature control circuit employs two such voltage dividers. The variable resistors are temperature set-point controls and the thermistors are ambient temperature sensors. A pair of diodes facilitate independent operation of the voltage dividers by isolating the junction points supplying voltage to the gate of the silicon controlled rectifier (SCR). An integrated-circuit clock is an integral part of the thermostat. This clock controls the application of voltage to the voltage dividers. Depending on the high or low voltage state of the alarm output line of the clock, a transistor circuit applies voltage to one or the other of the voltage dividers and, thus, selects the set-point temperature to be maintained by the thermostat. By setting the clock to alarm at certain times and setting the length of time during which the alarm state is to continue, the selection of the set point temperature to be maintained is made time dependent.
A smoke detector is also an integral part of the thermostat. It disables the operation of the thermostat in the presence of smoke by turning on a second silicon controlled rectifier which removes voltage from the gate of the power control SCR. The principals upon which the smoke detector operate involve a cylindrical smoke chamber having highly reflective inner surfaces with the exception of one end which is flat black. Light from a source located inside is reflected by the inner surfaces of the chamber through out its length resulting in a high light flux density. A photodarlington transistor is mounted in the chamber. The reflected light does not normally impinge the photodarlington but smoke entering the chamber scatters some the the light onto its base detector. The light flux has direction nearly perpendicular to the face of the base detector of the photodarlingron. Its high flux density facilitates the detection of small amounts of smoke. Refracted light impinging the photodarlington causes fluctuations in current flow through it and, thereby, increases the voltage applied to the gate of the silicon controlled rectifier which disables the thermostat.
The thermostat operates on a continuous, small, leakage current-flow from the power source through the heat exchanger to which full power is controlled. A charge is maintained in a battery which supplies power to the thermostat when full power is being applied to the heat exchanger. A pulse transformer located in series with the power source uses the always present current flow through the heat exchanger to supply the 60/50 Hz syncronizing clock pulses needed by the integrated-circuit clock.
An object of the present invention is to provide a timer-controlled thermostat which can be installed by a user who is not a skilled electrician. This has been accomplished by the design of the present thermostat which uses the existing two hook-up wires operating a replaced bimetallic-strip thermostat. It is capable of controlling the full operating power of a heat exchanger. It also can control the application of some intermediate power level to a load such as a relay which, in turn, switches full power to and from the heat exchanger.
A further object of the present invention is to provide a device which sounds an alarm and cuts off a heating or cooling unit in case of smoke. The resulting loss of air currents is intended to slow the rate at which a possibly present fire might spread.